In the present invention, the term restoration refers to the reconstitution of decayed teeth due to a shock, a viral or bacterial infection such as decays, especially the filling of cavities.
Amalgams and resin composites are still the most widely used materials for dental crowns restoration because of their excellent mechanical properties and their easy application. Amalgams allow a period of closure on average of 14 years, which is however reduced to only about 7 years for resins composites. Even used for a long time to restore dental crowns, silver amalgams that include mercury in their formulations cause problems with patient safety in case of release in the saliva. Moreover, their metallic appearance is unsightly. Resins composites that have been proposed as an alternative to silver amalgam to solve the aesthetic problem, present a short effective setting time.
The glass ionomer cements which have also been developed, proposed a possible aesthetical solution, but have disadvantages that have generally limited their use for the filling of teeth: it is impossible to place them in direct contact with the pulp tissue and strength is limited.
Obtaining improved mechanical properties, including good mechanical resistance to compression, longer life and excellent biocompatibility, is a challenge in the field of dental compositions. In fact, the dental compositions of the prior art, using Portland cement, have good biological properties but relatively poor mechanical properties.
In patent application FR 10 52631 filed on Apr. 7, 2010, the Applicant proposed improved calcium silicate based cements for dental restoration. Compositions disclosed in this patent application comprise calcium silicate crystals and calcium carbonate crystals with a specific ratio between the d50 granulometry of calcium silicate crystals and calcium carbonate crystals. This composition show an excellent biocompatibility, a good sealing, a short setting time and good mechanical properties with a compressive strength measured at one month of 300 MPa.
Improving wear resistance of dental material is a never ending issue in the art, and there always remain a need for cement compositions having improved mechanical properties. Especially, parameters that may be improved in order to obtain more sustainable restorations are gathered under the term “wear resistance” and include abrasion resistance, flexural strength, surface hardness, crack propagation and volumetric integrity of the material during compression. It is a prerequisite that additives that may be envisaged to improve these parameters should not affect working and setting times and compressive strength of the resulting composition. Moreover the consistency of the composition during working time should be controlled for easy handling by dental practitioner.
Among possible additives to be added in the composition of dental cements, the Applicant envisaged the addition of fibers in dental cements. Even though fibers were tested by their addition in resins as disclosed in international patent application WO2008/000917 or in U.S. Pat. No. 6,334,775, there are very few examples in the prior art of fibers in dental cements.
One example of prior art disclosure is U.S. Pat. No. 2,122,011, disclosing a dental cement comprising cellulose filaments treated to render them non-swelling and non-absorptive: in this patent application, cellulose fibers was claimed for improving the strength under impact of the dental cement, however the patent application does not bring any evidence related thereto, and the skilled artisan cannot find in the specification any test supporting this claim, and no data is made available with regards to compressive or flexural strength, wear resistance or crack propagation of the resulting material.
A second example of fibers in dental cements is disclosed in patent application US2008/0206716, wherein a bioactive endodontic material for filling dental cavities is disclosed, comprising calcium silicate, calcium phosphate and calcium oxide. Among a list of diverse additives, resorbable or non-resorbable fibers may be added. However, type, geometry and proportions of these fibers are not mentioned and no effect of this addition is showed or claimed.
Patent application US2008/0299093 discloses a premixed cement paste for use in medical or dental applications. The composition comprises a calcium silicate compound and a water-free liquid carrier. The cement paste remains liquid when hermetically stocked and hardens when placed in a physiological environment. This cement may optionally comprise fibrous materials such as poly-lactic acid biopolymer or poly-lactic-co-glycolic acid, aiming at enhancing physical and mechanical properties. However, the size and proportions of these fibrous additives is not specified.
It appeared to the Applicant that the prior art was poor in showing the interest and use of fibers in dental cements, and it conducted further research in order to understand how fibers, their particular geometries and their use in selected amounts proportions could really enhance wear resistance of dental cements.
In the same time, the Applicant conducted parallel research on the addition of porous fillers in dental cements. At the date of this invention, the use of porous particles was overwhelmingly disclosed in resins.
Especially, patent application US2005/025622 discloses a dental resin in which ceramic fillers of customized shape are added to improve fracture toughness performance and resistance to abrasion. These particles may be porous hollow spheres. The improvement of the mechanical properties is supposed to be linked to the mechanical interlocking of a ceramic particle with customized shape within the resin matrix.
Also, U.S. Pat. No. 4,381,918 discloses a composite of organic resin and organic porous particles for use in a dental restoration material. In this patent, inorganic porous particles are impregnated with the hardenable resin material by applying pressure. The resin penetrates through the pores of the particles leading to the binding of the particles by the hardening of the resin. The resulting structure therefore includes a contiguous inorganic phase within a resin matrix. The resulting material is reported to have good mechanical and physical properties, especially hardness.
As a professional in dental compositions, the Applicant is fully aware that an additive of interest in resins may not be adapted for cements. Despite thorough investigations in the prior art, and despite the fact that the above disclosures are now ancient, the Applicant could not find in the prior art any data related on the effect of the addition of porous fillers in dental cements.
The only prior art found relative to the use of porous fillers in non-resins dental filling material is patent application US2003/121455. This US patent application discloses a raw compact for dental purpose comprising calcium aluminate as main binding phase and expansion-compensating additives that may be porous aggregates. In this application, the solid components are mixed, granulated and compacted to form a raw compact. A ceramic material is obtained by means of first partially hydrating the raw compact with a hydration liquid and then placing it into a cavity of a tooth that needs restoration. Due to in situ complete saturation in connection with saliva secretion, the material hardens to provide ceramic material. During hydration, the raw compact undergoes an expansion and fills the tooth cavity. However, if the expansion is too important, tooth breakage may occur. The presence of porous aggregates intends to compensate expansion and thus take up inner stresses caused by dimensional changes.
Above prior art relates to a raw compact but not to a dental cement. Therefore, the only way to have information on the effect of the addition of porous fillers in dental cements was to conduct a specific research work program on this topic, which is what the Applicant did.
Surprisingly, the Applicant found that adding fibers or porous fillers or a combination thereof in a dental cement composition could lead to improved mechanical properties. Especially the present invention proposes a dental cement composition, which is resistant to abrasion and flexion, and wherein the crack propagation is limited, the surface hardness is improved and the material maintains its integrity during compression. The present invention also relates to means for manufacturing such composition.